The present invention relates to a distance measuring apparatus using a diffractive optical element (hereinafter abbreviated to "DOE") with a diffraction surface having a lens action based on a diffraction phenomenon. More particularly, the present invention relates to an optical system used in an active distance measuring apparatus that measures the distance to a subject by projecting infrared light onto the subject and receiving reflected light therefrom.
An active distance measuring apparatus that measures the distance to a subject by projecting infrared light onto the subject and detecting reflected light therefrom has heretofore been well known and already applied to various products. In this type of distance measuring apparatus, infrared light is projected onto a subject through a light-projecting lens system, and reflected light from the subject is received by a detector through a light-receiving lens system provided apart from the light-projecting lens system by a predetermined distance, that is, baselength. Then, the distance to the subject is calculated from position information on the detector.
The active distance measuring apparatus will be described below more specifically with reference to FIG. 1. FIG. 1 is a block diagram showing an essential part of an active distance measuring apparatus. An infrared-emitting diode (hereinafter referred to as "IRED") 11 is controlled by a control unit 11a. A light-projecting lens system 12 projects light emitted from the IRED 11 onto a subject 13. Reflected light from the subject 13 is received and converged through a light-receiving lens system 14. A position sensing device (hereinafter referred to as "PSD") 15 detects the position of the converged light. Reference numeral 16 denotes a distance calculating device. Reference numeral 17 denotes a controller that performs a position calculation for a focusing lens. Reference numeral 18 denotes a driver, and reference numeral 19 denotes a driving motor. Examples of the IRED 11 include an IRED in which a light-emitting portion is covered with a resin package having a curvature as shown in the figure, and an IRED in which a light-emitting portion is covered with a resin package having a plane surface. The controller 17 incorporates a CPU. The output of the CPU drives the motor 19 as a power source for the lens focusing motion produced by the driver 18.
In the distance measuring apparatus having the arrangement as shown in FIG. 1, assuming that the subject distance is d, the distance between the light-projecting lens system 12 and the light-receiving lens system 14, that is, baselength, W, the focal length of the light-receiving lens system 14 is f, and the position of light converged on the PSD 15 is x, the following relationship is obtained: EQU d=W.multidot.f/x (a)
Assuming that photoelectric currents outputted from both sides of the PSD 15 are I.sub.1 and I.sub.2, the ratio I.sub.1 /I.sub.2 is not dependent on the intensity of incident light but determined by only the incident light position x. Assuming that the entire length of the PSD 15 is t, the following relationship is obtained: EQU I.sub.1 /I.sub.2 ={(t/2)+x}/{(t/2)-x} (b)
From the above Eqs.(a) and (b), the following relationship is obtained: EQU I.sub.1 /I.sub.2 ={t+(2.multidot.W.multidot.f/d)}/{t-(2.multidot.W.multidot.f/d)}(c)
Accordingly, if the photoelectric current ratio I.sub.1 /I.sub.2 of the PSD 15 is obtained, the subject distance d is uniquely determined.
The distance measuring apparatus of the type described above is based on the trigonometrical measurement. In a case where the apparatus has a distance measurement range only in the center of the image field, if the principal subject is not in the center of the image field, the distance measuring apparatus is undesirably focused on another subject or a background (i.e. a distant object in many cases). Consequently, a focusing error occurs, resulting in a picture in which the principal subject is out of focus. To overcome such a disadvantage, a technique known as multipoint distance measurement has been proposed in which a plurality of light beams are projected to enable distance measurement to be performed in a plurality of ranges in the image field. See Japanese Patent Application Unexamined Publication Number hereinafter referred to as "JP(A)"! 4-248509 as an example of a light-projecting lens system. As specific examples of this technique, the following methods are known: A method in which a plurality of light-emitting units are prepared, and a plurality of light beams are projected through a single lens; and a method in which a plurality of light beams are obtained from a single light-emitting unit through a lens divided into a plurality of surfaces having different curvatures. In the case of a fixed (single) focal length lens, the phenomenon in which the principal subject that is not in the center of the image field undesirably becomes defocused can be satisfactorily prevented by such a conventional method. However, in the case of a camera that effects zooming or a magnification change such as switching between a telephoto position and a wide-angle position, the field angle changes. Therefore, it is necessary to change the angle of projected light (i.e. the angle formed between the central beam of the projected light and the peripheral beam of the projected light) in accordance with the change of the field angle. To meet such a demand, a method in which a large number (5 or 7) of light beams are prepared and selectively projected has also bee proposed.
Meanwhile, JP(A) 63-292118 is known as an example in which the light-projecting lens system also effects a magnification change in accordance with a magnification change of the taking lens. However, this prior art is not adapted for the multipoint distance measurement but schemed to change the size of the spot of projected light in accordance with the magnification change of the taking lens so that the range of a frame for distance measurement displayed in the viewfinder is always coincident with the range of the actual spot. More specifically, a zoom lens system consisting essentially of two, negative and positive, lens units is used as a device for changing the size of the spot of projected light, and the focal length is changed by changing the spacing between the two lens units. In JP(A) 6-94976, spots of projected light for multipoint distance measurement line up in the diagonal direction of the film format, and when panoramic photography is performed, the angle of projected light is reduced so that the projected light beams will not be blocked by the panoramic mask. As a device for changing the projected light angle, a cylindrical lens is inserted after the light-projecting lens. With this method, however, the projected light angle can be changed only in a predetermined direction.
Thus, there is no prior art relating to a scheme of adapting the light-projecting lens system or the light-receiving lens to achieve a magnification change, and the subject matter of the related art is not satisfactory. However, there has recently been a growing tendency for cameras to have a zooming function. Therefore, there is an increasing need for a distance measuring apparatus capable of varying the projected light angle.
Next, diffractive optical elements (DOEs) will be described. Regarding the application of DOEs, "Hybrid diffractive-refractive lenses and achromats" Appl. Opt. 27,2960-2971, "International Lens Design Conference (1990)" SPIE, 1354, etc. are known.
Conventional lenses are based on the refracting action at the interface of a medium, whereas DOEs are based on the diffracting action of light. In general, when light enters a diffraction grating as shown in FIG. 2, diffracted light emanating from the diffraction grating satisfies the following relationship: EQU sin.theta.-sin.theta.'=m.lambda./d (d)
where .theta. is the incident angle; .theta.' is the exit angle; .lambda. is the wavelength of light; d is the pitch of the diffraction grating; m is the order of diffraction.
Accordingly, if the pitch of the ring-shaped diffraction grating is appropriately set according to Eq.(d), the incident light can be converged on a point. That is, a lens action can be given to the diffraction grating. Assuming that the radius of the j-th grating ring is r.sub.j and the focal length of the diffraction surface is f, the following condition is satisfied in the domain of linear approximation: EQU r.sub.j.sup.2 =2j.lambda.f (e)
Examples of diffraction grating configurations hitherto proposed include an amplitude modulation type in which a diffraction grating is formed from bright and dark rings, and a phase modulation type in which the refractive index or the optical path length is varied. In the amplitude modulation type DOE, a plurality of orders of diffracted light are generated; therefore, the ratio of the amount of first-order diffracted light to the amount of incident light (hereinafter referred to as "diffraction efficiency"), for example, is about 6% at the most. Even if the amplitude modulation type DOE is improved by bleaching, the diffraction efficiency is about 34% at the most. In the phase modulation type DOE, the diffraction efficiency can be increased up to 100% if it is formed with a sawtooth sectional configuration such as that shown in FIG. 3(a). Such a DOE is known as "kinoform". In this case, the height of the crests of the sawtooth sectional configuration is given by EQU h=m.lambda./(n-1) (f)
where h is the height of the crests, and n is the refractive index of the base material.
As will be predicted from Eq.(f), the diffraction efficiency 100% can be attained for only one wavelength. An optical element formed by stepwise approximation of the kinoform configuration as shown in FIG. 3(b) is also known as "binary optical element", which can be produced relatively easily by a lithographic technique. In the case of binary optical elements, it is known that a diffraction efficiency of 81% is obtained by 4-step approximation; 95% by 8-step approximation; and 99% by 16-step approximation.
Some DOE designing methods are known. In the present invention, the ultra-high index method is used. This method is described, for example, in "Mathematical equivalence between a holographic optical element and ultra-high index lens" J. Opt. Sos. Am. 69,486-487, and "Using a conventional optical design program to design holographic optical elements" Opt. Eng. 19,649-653. In other words, it is known that a DOE is equivalent to a refracting surface having a thickness of zero and an exceedingly large refractive index.
As an example in which such a DOE is applied to an active distance measuring apparatus, JP(A) 7-63982, filed by the present applicant, is known. In this publication, a converter lens is inserted on the IRED side of a master lens to effect a magnification change. In this case, the principal point position of the converter lens is appropriately set to enable a magnification change to be effected with the master lens remaining fixed. This cannot be done by only a conventional refracting lens system from the viewpoint of aberration correction. Regarding the lenses in this publication, the master lens is a convex-plano lens, and the converter lens is a concave-plano lens. The plane surface of each lens is formed from a diffraction surface.
In JP(A) 7-63982, however, each lens is unfavorably thick because it is formed by adding a diffraction surface to a lens with a surface having a large curvature. Therefore, it is impossible to achieve a reduction in the size. The intensity of light weakens in inverse proportion to the square of the distance. In the case of the active type, the subject is illuminated by a certain light beam, and reflected light from the subject is detected. Therefore, as the distance to the subject increases, the intensity of reflected light rapidly weakens. However, under the necessity of performing distance measurement in the light during the day, infrared light is employed in order to use a signal light for distance measurement at a low noise level. Accordingly, a sufficiently high brightness is demanded of light for distance measurement. Consequently, the light-projecting and -receiving lenses are demanded to have a light-passing ability of about 1 in terms of F-number. To meet such a demand, the lenses inevitably become thick. However, there is a recent tendency for cameras to become compact. Therefore, the lens systems for distance measurement are demanded to reduce in size.